An aircraft may be provided with an ullage passivation system that may be used to render fuel tank gases within the aircraft substantially inert. The ullage passivation system is employed to reduce the possibility of a thermal event within the fuel tank. Ullage passivation systems are continuously operated on commercial and military aircraft to maintain the ullage gas mixture below lean flammability limits during all phases of aircraft flight. The continuous operation of the ullage passivation system may increase fuel costs of the aircraft.